XML is a subset of Standard General Markup Language (or something real close to that). SGML is a nasty thing, but I'd bet there are grammars and validators out there.
Roll your own could be fun. You can define grammars and make parsers to validate them. It's a whole other world when you get into that stuff ... I wouldn't try to become expert in it unless I thought there was a career waiting. Look at ANTLR for a start.
I'm curious why you're asking? [ January 25, 2005: Message edited by: Stan James ]
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi